Pedestal configuration for spinning low-viscosity melts

ABSTRACT

THE APPARATUS OF THIS INVENTION FOR SPINNING LOW VISCOSITY MELTS INCLUDES A CRUCIBLE HAVING AN ESSENTIALLY PLANAR, LOWER LAPPED BASE SURFACE AND A PEDESTAL HAVING AN ESSENTIALLY PLANAR, UPPER LAPPED SUPPORT SURFACE WITH THE CRUCIBLE BEING POSITIONED ON THE PEDESTAL SO THAT THE LAPPED SURFACES ARE IN CONTIGUOUS CONTACT. THE PEDESTAL GENERALLY DEFINES AN OPEN-ENDED CAVITY AND THE CRUCIBLE INCLUDES AN ORIFICE THROUGH WHICH THE LOW VISCOSITY MELT STREAMS, THE STREAM OF MELT BEING STABILIZED IN THE OPEN-ENDED CAVITY   BY THE FORMATION THEREAROUND OF A STABILIZING FILM. A PRESSURE DIFFERENCE IS CREATED ACROSS THE CONTIGUOUS LAPPED SURFACES IN ORDER TO CAUSE EXTRUSION OF THE LOW VISCOSITY MELT WITH THE LAPPED SURFACES BEING EFFECTIVE TO CREATE A SEAL AND PREVENT THE LEAKAGE OF GAS FROM THE ATMOSPHERE SURROUNDING THE CRUCIBLE INTO THE ATMOSPHERE CONTAINED IN THE PEDESTAL CAVITY.

June 8, 1971 c, GARRETT ETAL 3,583,027

PEDESTAL CONFIGURATION FOR SPINNING LOW-VISCOSITY MELTS Filed NOV. 15, 1968 ROBERT E; CUNNINGHAM LEWIS C. GARRETT INVENTORS ATTORNEYS United States Patent 01 lice 3,583,027 PEDESTAL CONFIGURATION FOR SPINNING LOW-VISCOSITY MELTS Lewis C. Garrett and Robert E. Cunningham, Raleigh,

N.C., assignors to Monsanto Company, St. Louis, Mo.

Filed Nov. 15. 1968, Ser. No. 776,086 Int. Cl. D01d 5/00 U.S. Cl. 188 1 Claim ABSTRACT OF THE DISCLOSURE The apparatus of this invention for spinning low viscosity melts includes a crucible having an essentially planar, lower lapped base surface and a pedestal having an essentially planar, upper lapped support surface with the crucible being positioned on the pedestal so that the lapped surfaces are in contiguous contact. The pedestal generally defines an open-ended cavity and the crucible includes an orifice through which the low viscosity melt streams, the stream of melt being stabilized in the open-ended cavity by the formation therearound of a stabilizing film. A pressure dilference is created across the contiguous lapped surfaces in order to cause extrusion of the low viscosity melt with the lapped surfaces being eifective to create a seal and prevent the leakage of gas from the atmosphere surrounding the crucible into the atmosphere contained in the pedestal cavity.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a low viscosity melt spinning apparatus and, more specifically, to an apparatus which is comprised of a pedestal and a crucible with the pedestal and crucible having contiguous essentially planar, and lapped surfaces which prevent the leakage of gas from an area surrounding the crucible into a gas reactive area which forms a stabilizing film about the streaming melt.

(2) Description of the prior art The spinning of filamentary articles from low viscosity melt materials has been beset with problems relating to the provision of an effective gas-type seal between melt crucible and its support pedestal. The failure to provide an effective gas-type seal between the melt crucible and the pedestal permits the leakage of the inert gas which blankets the crucible into the free jet stream film stabilization zone. The presence of the inert gas in the film stabilization zone may retard or prevent the formation of a stabilizing film along the free jet stream surface. Without the stabilizing film temporarily containing the free stream melt in fiber form, the surface tension of the melt causes the free stream to break up prior to solidification and form shot.

Heretofore, the crucible was mounted on the pedestal with the bottom of the crucible resting on a knife edge which formed an integral part of the pedestal. This arrangement had two particular disadvantages in that the knife edge had to be perfectly machined if an effective gas-type seal was to be maintained and the entire area represented by the bottom of the crucible was exposed to relatively cool stabilizing film gases which resulted in thermal stresses and ultimately cause a crucible failure.

3,583,027 Patented June 8, 1971 The large exposed area also radiated heat which resulted in significant temperature gradients and caused inconsistencies in the streaming of the melt.

SUMMARY OF THE INVENTION The apparatus of this invention. for spinning low viscosity melts includes a pedestal which generally defines an open-ended cavity containing a reactive gaseous atmosphere and a crucible mounted on the pedestal which is blanketed by a separate inert gaseous atmosphere. The pedestal has a lapped substantially horizontal upper surface which is provided with an opening that extends to and is in communication with the pedestal cavity. The upper surface may also be provided with a groove which is situated about the surface opening and, for example, may be annular in shape and concentrically arranged about the surface opening. Where a groove is provided, a passageway extends from the groove to the cavity so that any gases trapped by a groove are transferred to the cavity and to a locus remote from the opening in the pedestal.

The crucible is also provided with a lapped base surface which is adapted to be received by and is in contiguous contact with the pedestal lapped surface. The two contiguous surfaces are so machined or lapped that an effective gaseous seal is created therebetween which normally prevents admission of a gas between the two surfaces and into the pedestal surface opening. The crucible contains an orifice through which the low viscosity melt streams. The orifice is of a smaller diameter than the pedestal surface opening so that the stream melt may pass through the pedestal opening and into the open-ended pedestal cavity. When reference herein is made to the orifice, the orifice is to include the actual hole through which the melt passes; however, the orifice itself may be drilled directly into the base of the crucible or the crucible may be adapted to receive an orifice insert. Thus, while the orifice insert may be larger than the pedestal surface opening, the actual orifice diameter is smaller than the pedestal surface opening.

The crucible is place in an enclosed area, is surrounded by an inert gas and is maintained under a positive pressure. After receiving a charge, the crucible is then heated by means of an induction heating coil and susceptor or some other like apparatus to melt the charge. The positive pressure enables the melt to stream through the small orifice opening at speeds which enable the process to be economically feasible. The pedestal cavity contains a film forming atmosphere which is generally at room temperature and pressure. This atmosphere acts to form a film about the free streaming melt and thus should be free to contact the stream within a short distance from the orifice. Since the atmosphere surrounding the crucible is inert and generally comprised of argon or such like gas, blanketing of a substantial volume below the orifice by the inert gas would disrupt the film forming capabilities of the reactive gas contained beneath the pedestal opening.

To make the seal existing between the two lapped surfaces more functional, the groove and connecting passageway is adapted to remove any gases which may leak between the surfaces to an area away from the orifice and pedestal opening. The leakage of small amounts of gases appears inherent because the crucible and the pedestal are generally made of different materials and thus expand at difiierent rates during the heating up of the apparatus for melting and streaming.

The crucible is generally made from a ceramic or other refractory material which is capable of withstanding extreme heat while the pedestal may be comprised of pyrolytic graphite because of its excellent anisotropic heat transfer properties. The graphite is to be arranged to preferentially transmit heat normal with respect to the flow of the stream of melt so as to provide an insulation to the crucible in other directions. The use of pyro lytic graphite effectively insulates the crucible base and eliminates most of the thermal stresses which would be expected to be created due to the large temperature gradients across the base.

Therefore, an object of this invention is to provide contiguous lapped surfaces of a crucible and its supporting pedestal whereby the necessary high pressure differentials can be used without exceeding the strength of the crucible material.

Another object of this invention is to provide an efl ective seal between the contiguous surfaces of a low viscosity melt spinning crucible and its support pedestal for preventing the leakage of pressurized gases from an area blanketing the crucible into a film forming area defined by the pedestal.

A further object of this invention is to provide a pedestal having improved heat transfer properties so as to reduce the temperature gradient across the bottom portion of a melt crucible to thereby prevent failure of the crucible.

DESCRIPTION OF THE DRAWING The figure is a. section view taken along the vertical axis of the apparatus and showing the relat1onsh1p of the various components comprising the apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the figure, crucible is supported by pedestal 11 with the bottom of pedestal 11 being sealed to a pressure plate (not shown) which is located at the bottom of the spinning head. Top surface 12 of pedestal plate is essentially planar and lapped fiat to receive crucible 10. Also, the exterior of base surface 13 of crucible 10 is likewise essentially planar and lapped flat so that upon placing crucible 10 on the pedestal 11 an air tight seal is formed therebetween. Pedestal 11 as shown includes walls 14 which are connected as a contiguous assembly to horizontally positioned pedestal plate 15. Plate 15 and walls 14 form open-ended cavity 16 which is generally filled with a film forming atmosphere such as air, ammonia, propane or the like which is adaptable to form the film about a free streaming melt with the particular gas being dependent upon the characteristics of the melt. Plate 15 is provided with a conically shaped pedestal opening 17 which is positioned along the vertical axis of pedestal 11. Crucible 10 is provided with an orifice 18 which is located in base 13 and through which melt 9 streams, orifice 18 being disposed above pedestal opening 17. The narrow portion of pedestal Opening 17 may be adapted to be only slightly larger than the diameter of orifice 18 so as to provide as much support and as much insulation as possible for base 13 of crucible 10. The mentioned support allows the use of large pressure differentials across orifice 18 such pressure being necessary for spinning of fine diameter filaments. The fiat lapped seal created by the placement and contiguous relationship of crucible 10 on pedestal 11 allows the respective components to move relative each other during the heating and cooling cycles since the coefficient of thermal expansion of crucible 10 is different from the coefficient of thermal expansion of pedestal 11.

Some leakage through the lapped fiat seal between pedestal 11 and crucible 10 and into pedestal opening 17 is almost inevitable where high temperatures are used to create the melt. Thus, provisions are made for removal of these gases which would normally accumulate in opening 17 and retard or prevent the formation of a film about the free streaming melt issuing from orifice 18 into opening 17. Since the continuity of the free streaming melt prior to film formation can be maintained for only a short distance which is frequently less than an inch, the length depending on jet diameter, surface tension, density and spinning velocity, the stabilizing film must be formed about the free streaming melt within a short distance in order to produce a stabilized molten stream. Therefore, an adequate concentration of the film forming gas in opening 17 is extremely important and leakage of an inert gas from the area surrounding crucible 10 into opening 17 could effectively prevent the film formation within a critical distance. To insure that the inert gas does not retard film formation to the extent that break up occurs prior to stabilization of the continuous molten stream, surface 12 of pedestal plate 15 is provided with a groove 19 which is depicted as annular in shape and concentrically arranged about the vertical axis of pedestal 11. The gases which collect in groove 19 are vented to pedestal cavity 16 by means of passageway 20, or they may be vented directly to the atmosphere. Thus, the entire pressure differential between the atmosphere blanketing crucible 10 and the reactive atmosphere contained by cavity 16 is taken across the lapped flat seal between crucible 10 and pedestal 11 and any gas leaking through that seal is removed through groove 19 and passageway 20 into an area of cavity 16 which is away from opening 17. Additionally, the cavity 16 may be vented to remove such inert gas.

Pedestal 11 may be provided with one or more heat shields 21 which are concentrically arranged within pedestal 11 and are provided with an opening 22 through which formed fiber 23 passes. Heat shield 21 functions to reduce the loss of heat from the bottom of pedestal plate 15 and also to provide a means for removing the vented inert gas. Therefore, an exhaust means (not shown) may be connected between heat shield 21 and pedestal 11 to aid in the removal of the vented gas.

The apparatus further includes an induction heating coil 25 and a susceptor 26 which is separated from induction heating coil 25 by means of insulation 27. Susceptor 26 rests on pedestal 11 by being seated in annular flange 29'. Susceptor 26- is provided with lid 31 which includes opening 32, opening 32 adapting crucible 10 to be charged without having to remove top 31. An annular spacing ring 33 may be provided which has an outer diameter substantially the same as inner diameter of susceptor and an inner diameter substantially the same as the outer diameter of crucible 10 so that orifice 18 may be placed directly over opening 17.

Certain variations are contemplated within the apparatus of this invention such as either of the contiguous surfaces may be considerably larger than the other. Further, the size of groove 19 can vary from a few thousandths of an inch wide to a very large size and in any event should 'be large enough to provide an easy path for gas leaking into that area to be vented through passageway 20 into cavity 16. Further, a plurality of passageways 20 may be provided where necessary. Although the apparatus of this invention has been illustrated as using only one orifice, it is contemplated that this apparatus is adaptable to include multiplicity of orifices or orifice inserts inside the area defined by the groove 19. Also, one pedestal opening 17 may be provided for each orifice 18 or an enlarged passageway 17 be provided for a plurality of orifices 18 depending upon the material being spun into fibers and the end use to which the fibers are to be placed.

As illustrated, many modifications of this invention may be made without departing from the scope and spirit thereof. Therefore, applicants do not wish to be bound except as what limitations appear in the appended claims.

What is claimed is: cavity at a position below said conically-shaped upper 1. An improved apparatus for spinning a low viscosity region thereby providing a path for any inert gas melt including a susceptor, a crucible enclosed by said leaking between said lapped regions to aposition away susceptor having a base with an orifice, a pedestal with a from said zone.

cavity which has an opening adjacent said orifice, said pedestal being comprised of a material having a different References Cited coefficient of expansion than said crucible, said pedestal UNITED STATES PATENTS further having an upper surface encircling said opening with said upper surface supporting said susceptor and said i i 5 crucible along the lower surface of said base, said crucible 41 10/1931 :32 lg '5 5 containing a low viscosity melt which under pressure of Ont an inert gas contained within said susceptor is streamed FOREIGN PATENTS through said orifice into a zone within said cavity near 595 308 3/1960 Canada. said orifice which zone contains a reactive gas at a pressure less than the inert gas pressure, said improvement OTHER REFERENCES Comprising Pyrolytic Graphite, Engineering Handb0ok-General (a) said lower surface of said crucible base and said Electric, July 15, 1953 upper surface of said pedestal being essentially planar and lapped in areas of contact; RICHARD J. HERBST, Primary Examiner (b) said cavity havinga substantially comically-shaped A. L HAVIS, Assistant Examiner upper region wherein said openlng of said upper region is larger than said orifice; CL (c) said upper surface of said pedestal having a groove 264 176F spaced apart and encircling said opening; and (d) a passageway extending from said groove to said 

